High resolution non-contact interior profilometer

ABSTRACT

Apparatus and method for inspecting the interior surfaces of devices such as vessels having a single entry port. Laser energy is launched into the vessel, and the light reflected from the interior surfaces is interfered with reference laser energy to produce an interference pattern. This interference pattern is analyzed to reveal information about the condition of the interior surfaces of the device inspected.

The present invention generally relates to procedures for inspecting the condition of manufactured articles, and, more particularly to the inspection of the interiors of manufactured articles such as tanks, medical implants, turbines, and other closed applications in which only a limited access is provided. This invention was made with Government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Inspection of manufactured articles is of great importance, particularly to manufacturers of critical equipment whose failure can produce catastrophic results. However, some of the articles in this category defy close inspection because the geometry of the article makes inspection extremely difficult for conventional inspection techniques.

Fiber optical devices are extensively used to view objects that normally would be considered inaccessible. The optical fiber elements for this purpose are typically smaller than 50 μm in diameter, including protective layers. Optical fibers such as these can be bent into radii as short as 3 cm, allowing their infiltration into areas that normally preclude direct imaging techniques.

The medical community has made extensive us of such optical fibers for endoscopic applications, such as the real-time imaging of internal organs to provide guidance for microsurgical techniques. Fiber-optic technology also has been applied in many other areas, such as opto-mechanical applications for numerous industrial and medical applications.

The primary problem with prior art, fiber optic imaging is that it is difficult to achieve acceptable measurement of surface tolerances or roughness through a single port or single optical fiber. Conventional fiber optic imaging also does not provide any quantitative measurement of the interior region being imaged. This is because prior art systems generally must rely on interferometry concepts requiring multiple fibers.

It is therefore an object of the present invention to provide profilometer apparatus for inspecting the interior surfaces of tanks and other device under tests, including medical implants and any enclosure having a single access port.

It is therefore an object of the present invention to provide apparatus for inspecting interiors of tanks and other device under tests that is capable of accurate quantitative measurement of interior surface imperfections with a high degree of resolution.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, apparatus for inspecting the interior of a device under test through a single port in the device under test comprising laser means for producing laser energy, with beam splitter means for directing the laser energy to a first direction and to a second direction. Delay means receive the laser energy from the second direction for introducing a predetermined delay to the laser energy from the second direction and outputting the delayed laser energy from the second direction. Optical routing means receive the laser energy from the first direction of at a first input for directing the laser energy from the first direction to ones of a first at least one optical fibers that enter the device under test through the port, and for transmitting laser energy reflected from interior surfaces of the device under test to ones of a second at least one optical fibers. Interferometer means receive the laser energy reflected from the interior surfaces of the device under test and the delayed laser energy from the second direction for interfering said laser energy reflected from the interior surfaces of the device under test with the delayed laser energy from the second direction and outputting interference patterns, wherein the interference patterns are representative of the interior surfaces of the device under test.

In a further aspect of the present invention, and in accordance with its objects and principles, a method for inspecting the interior surfaces of a device under test having a single entry port comprises the steps of launching laser energy into the device under test; receiving light reflected from the interior surfaces of the device under test; interfering the light reflected from the interior surfaces of the device under test with reference laser energy, creating an interference pattern; analyzing the interference pattern to discern information about the interior surfaces of the device under test; and outputting the information about the interior surfaces of the device under test.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the components of the present invention.

FIG. 2 is a schematical illustration of one of the interferomet units for output from the present invention.

FIG. 3 is a side view detail of the end of three optical fibers inserted into a device under test to be inspected so that upon rotation and translation of the device under test virtually all of the interior suffice can be inspected.

FIG. 4 is a block diagram of the control portion of the present invention and its interconnection to other control devices.

DETAILED DESCRIPTION

The present invention provides apparatus for the high resolution inspection of the interior of device under tests, having the capability of such inspection even should the device under test only have a single port of access. The invention can be understood most easily through reference to the drawings.

In FIG. 1, a block diagram of the components of the present invention is illustrated. As seen, laser system 11, which may be a Titanium doped Sapphire laser pumped by an Argon ion laser outputting approximately 30-100 femtosecond (10⁻¹³ seconds) pulses to beamsplitter 12, which divides the pulse output of laser system 11 between optical path 12 a and optical path 12 b. However, as will be discussed below, many forms of laser energy with different modulation techniques can be used with the present invention. Optical path 12 a carries a portion of the laser energy of laser system 11 to optical routing module 13. Optical fiber 12 b carries the remaining portion of the laser energy output of laser system 11 to mirror 14 by which the correlated output of laser system 11 is directed into correlation module 15.

As will hereinafter be more particularly described, optical routing module 13 splits the pulses from beamsplitter 12 and directs them into inspection fibers 13 a, 13 b, and 13 c which are inserted into device under test 16. Device under test 16 is mounted onto fixture 17, which is capable of translating and rotating device under test 16, and which is itself controlled by fixture and laser control unit 18 a. Fixture and laser control unit 18 a indexes the rotational movement of fixture 17 and inserts inspection fibers 13 a, 13 b, and 13 c into device under test 16 by an amount commensurate with the desired cross-sectional resolution of the profile measurement. Fixture and laser control unit 18 a also controls the output of laser system 11, controlling such functions as pulse width and rate, or frequency (wavelength). The Fixture and laser control unit 18 a, is in turn under the control of main controller 18 d.

Optical routing module 13 serves several optical functions. Initially, optical routing module 13 directs the incident pulses output from laser system 11 into optical fibers 13 a, 13 b, and 13 c for transmission to device under test 16. Optical routing module 13 also directs pulses returning from device under test 16 on optical fibers 13 a, 13 b, and 13 c into optical fibers 13 d, 13 e, and 13 f respectively. Optical fibers 13 d, 13 e, and 13 f are routed to interferometer detectors 15 and to individual nonlinear crystals 15 a, 15 b, and 15 c, respectively.

As shown in FIG. 2, for an individual nonlinear crystal 15 a, 15 b, or 15 c of interferometer detectors 15, receive the signal reflected from the interior of device under test 16 (FIG. 1) which are on optical fibers 13 d, 13 e, and 13 f. Also input to nonlinear crystals 15 a, 15 b, or 15 c is the transmitted signal on optical fiber 12 b. The interference of the transmitted signal with the received signal produces interference pattern 22 that is magnified by magnification lens 23. The magnified interference pattern is input to CCD camera 18 b for conversion into an electrical signal representative of high resolution inspection of the interior surface of device under test 16.

In operation, non-linear optical crystals 15 a, 15 b and 15 c serve to convert the optical energy in the pulses arriving from the transmit reference fiber, 12 b and from the receive signals on optical fibers 13 a, 13 b, or 13 c into energy detectable by the Charge Coupled Device (CCD) 18 b. The pattern of optical energy observed in the non-linear crystals, 15 a, 15 band 15 c is a direct measure of the difference in total path difference between the transmit paththrough optical fibers 12 a, and 13 a, 13 b, and 13 c respectively, and the transmit reference path optical fiber 12 b.

Non-linear optical crystals 15 a, 15 b, and 15 c may be any appropriate non-linear optical crystal. Examples of suitable crystals of use in the present invention are Beta Barium Borate (BBO) and Potassium Dihydrogen Phosphate (KDP) crystals.

Turning now to FIG. 3, there can be seen optical fibers 13 a, 13 b, and 13 c entering device under test 16 through a single port 16 a. As shown, optical fibers 13 a and 13 c have their ends finished to form a 45° reflector, with a high reflective coating applied to surfaces 31 and 32, and an anti-reflecting coating applied to surfaces 33 and 34. These coatings will direct light traveling along optical fibers 13 a and 13 c at a right angle to the longitudinal axes of optical fibers 13 a and 13 c. The end of optical fiber 13 b is finished at a 90° angle, and has an anti-reflective coating applied to its surface 35, to direct light along its longitudinal axis. With optical fibers 13 a, 13 b, and 13 c so configured, virtually complete coverage of the interior surfaces of device under test 16 is possible when device under test 16 is rotated and laterally moved by fixture 17 (FIG. 1).

Returning now to FIG. 1, it should be noted that the output of laser system 11 is modulated in order to facilitate analyzation of the signals returning from the interior surface of device under test 16. This modulation, which is applied prior to output from laser system 11, can be chosen from the group consisting of short pulse, frequency modulated continuous wave (FMCW) or chirp, and stepped frequency phase measurement. Each type of modulation has its own advantages, with the particular type dependent on the particular application. However, any one of these modulation techniques should provide satisfactory results with the present invention.

As stated, each of these modulation techniques has its own advantages. Short pulse modulation offers excellent resolution and direct measurement, but is somewhat complex and the pulse resolution is inversely proportional to pulse width. FMCW or chirp modulation, a linear chirp over the same bandwidth as short pulse modulation, has its resolution limited only by the total bandwidth, and accomplishes range measurement in the frequency domain, but is disadvantaged by the possibility its wide bandwidth can produce RF signals, and by its complexity in developing a high linearity sweep. Finally, stepped frequency phase measurement provides discrete wavelength, and accurate phase measurement. Its advantages are that the resolution is limited only by the total wavelength, and that range measurement is in the phase domain. Its disadvantages include that its resolution is limited by phase stability, and its high complexity in accurately measuring phase at optical wavelengths.

It is to be understood that the particular type of modulation employed in practicing this invention will require analysis of the requirements so that the correct modulation for the application can be utilized. It is a question of the resolution required and the amount of complexity allowed in the application. In the invention, the modulation of laser system 11 is provided through fixture and laser control 18 a.

As illustrated for one interferometer in FIG. 2, interference pattern 22 is imaged onto CCD camera 18 b for each optical fiber 13 a, 13 b, and 13 c shown in FIG. 1. CCD camera 18 b provides its output to frame grabber and DSP (Digital Signal Processor) 18 c for output.

Main controller 18 d provides control to fixture and laser control 18 a and to frame grabber and DSP 18 c. The controller can be any general-purpose personal computer. Control bus 18 c provides the connection between frame grabber and DSP 18 c, main controller 18 d and fixture and laser control 18 to provide the necessary communication between these devices.

FIG. 4 is a block diagram of one embodiment of the computer control system for the present invention. As shown, fixture and laser control 18 a and frame grabber and DSP 18 c are connected to I/O bus 18 e. Central processing unit 24 also is connected to I/O bus 18 e as well as to removable hard drive 25, fixed hard drive 26, and display 27. Central processing unit 24 is programmed with the software necessary to provide the appropriate control signals to fixture and laser control 18 a and frame grabber and DSP 18 c, and to analyze and display the signals returned to it from the interior surfaces of device under test 16 (FIG. 1).

The present invention can find application in numerous important areas. The inspection of interior surfaces of important vessel assemblies can further reduce the danger of vessel rupture and the concomitant dangers and expense. Devices intended for implantation into human bodies likewise must be examined so that any possible leakage is discovered and corrected prior to implantation. Thus, the present invention may prove to be invaluable in the discovery of surface problems in any vessel that has limited entry ports.

The foregoing description of the embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto. 

What is claimed is:
 1. Apparatus for inspecting the interior of a device under test through a single port in the device under test comprising: laser means for producing laser energy; beam splitter means for directing said laser energy to a first direction and to a second direction; delay means receiving said laser energy from said second direction for introducing a predetermined delay to said laser energy from said second direction and outputting said delayed laser energy from said second direction; optical routing means receiving said laser energy from said first direction at a first input for directing said laser energy from said first direction to ones of a first at least one optical fibers that enter said device under test through said port, and for transmitting laser energy reflected from interior surfaces of said device under test to ones of a second at least one optical fibers; interferometer means receiving said laser energy reflected from said interior surfaces of said device under test and said delayed laser energy from said second direction for interfering said laser energy reflected from said interior surfaces of said device under test with said delayed laser energy from said second direction and outputting interference patterns; wherein said interference patterns are representative of said interior surfaces of said device under test.
 2. The apparatus as described in claim 1, further comprising fixture means for holding said device under test and for controllably positioning said device under test relative to said first plurality of optical fibers.
 3. The apparatus as described in claim 1, wherein said first and second at least one optical fibers each comprises three optical fibers.
 4. The apparatus as described in claim 3, wherein said first at least one optical fibers comprise three optical fibers wherein two of said three optical fibers have ends that are cut at 45° angle surfaces and direct light at a right angle with respect to said light traversing said two of said three optical fibers.
 5. The apparatus as described in claim 4, wherein said two optical fibers have reflective coating applied to said 45° angle surfaces.
 6. The apparatus as described in claim 1, further comprising camera means in optical contact with said interference patterns for imaging said interference patterns and outputting said image.
 7. The apparatus as described in claim 2, further comprising computer means receiving said image for displaying and analyzing said image, and for controlling said fixture means.
 8. The apparatus as described in claim 1 wherein said laser energy is modulated using short pulse modulation.
 9. The apparatus as described in claim 1 wherein said laser energy is modulated using frequency modulated continuous wave modulation.
 10. The apparatus as described in claim 1 wherein said laser energy is modulated using stepped frequency phase measurement modulation.
 11. The apparatus as described in claim 1 wherein said interference patterns are created in a nonlinear optical crystal in said interferometer means.
 12. The apparatus as described in claim 1 wherein said nonlinear optical crystal comprises a Beta Barium Borate crystal.
 13. The apparatus as described in claim 1 wherein said nonlinear optical crystal comprises a Potassium Dihydrogen Phosphate crystal. 